Water treatment process and water treatment system

ABSTRACT

Provided are a water treatment system and a water treatment process, which are capable of reproducing water containing salts with high water recovery. In the water treatment system ( 200 ) and the water treatment process of the present invention, after a calcium scale inhibitor and a silica scale inhibitor are supplied to water to be treated containing Ca ions, SO 4  ions, carbonate ions, and silica, and the water to be treated is separated in a second demineralizing section ( 210 ) into second concentrated water in which the Ca ions, the SO 4  ions, the carbonate ions, and the silica are concentrated and treated water. In a second crystallizing section ( 220 ), seed crystals of gypsum are supplied to the second concentrated water, whereby gypsum is crystallized and removed from the second concentrated water.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of International Application PCT/JP2014/064517, with an international filing date of May 30, 2014, which is hereby incorporated by reference herein in its entirety. This application claims the benefit of Japanese Patent Application No. 2013-141932, the content of which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a water treatment process and a water treatment system for reproducing water to be treated containing Ca ions (Ca²⁺), sulfate ions (SO₄ ²⁻), carbonate ions, and silica.

BACKGROUND ART

It is known that industrial waste water, saline water, and sewage contain large amounts of ions and silica. In addition, in a cooling tower, heat is exchanged between a high-temperature exhaust gas discharged from the boiler, etc., and cooling water. As a result of this heat exchange, some of the cooling water turns into steam, and, accordingly, ions and silica in the cooling water are concentrated. Therefore, the cooling water discharged from the cooling tower (blowdown water) has increased concentrations of ions and silica.

Water containing a large amount of ions is subjected to a demineralization treatment and then discharged into the environment. As devices that perform the demineralization treatment, a reverse osmosis membrane device, a nanofiltration membrane device, an ion-exchange equipment, and the like are known.

Among ions contained in the water mentioned above, monovalent cations such as Na⁺, K⁺, and NH₄ ⁺ and anions such as Cl⁻ and NO₃ ⁻ are highly soluble in water. On the other hand, divalent metal ions such as Ca²⁺, anions such as SO₄ ²⁻ and CO₃ ²⁻, and silica are scale-forming components. Salts and silica of scale-forming components have low solubility in water, and thus they tend to be deposited as scales. In particular, the saline water, industrial waste water, and blowdown water from a cooling tower mentioned above contain large amounts of Ca²⁺, SO₄ ²⁻, carbonate ions (CO₃ ²⁻, HCO₃ ⁻), and silica. An example of the property is as follows: pH: 8, Na ions: 20 mg/L, K ions: 5 mg/L, Ca ions: 50 mg/L, Mg ions: 15 mg/L, HCO₃ ions: 200 mg/L, Cl ions: 200 mg/L, SO₄ ions: 120 mg/L, PO₄ ions: 5 mg/L, SiO₂ ions: 35 mg/L. Among these, the concentrations of Ca ions, Mg ions, SO₄ ions, and HCO₃ ions are high, and as a result of their reaction, scales (CaSO₄, CaCO₃, etc.) are formed. In addition, depending on the concentration percentage, silica components present in waste water also serve as scale components adhering to the instrument, etc. When scales are produced in the device that performs a demineralization treatment, the treatment capacity is reduced. Therefore, it is required to perform a demineralization treatment without allowing for the production of scales.

Here, examples of plants using a water-cooling-type cooling tower are plants equipped with power generation facilities (power generation facilities include those for business purposes for electric power selling and those for industrial purposes for in-house electricity use, and the power generation is thermal power generation, geothermal power generation, etc.), plants equipped with power generation facilities and cooling facilities, etc. In addition, plants include ordinary chemical plants, steel plants, mining plants, oil field plants, gas field plants, mechanical plants, etc.

As a process for removing Ca ions, a lime soda process is known. According to the lime soda process, sodium carbonate is added to water to be treated, and Ca ions in the water to be treated are deposited/precipitated as calcium carbonate and thereby removed from the water.

Patent Literature 1 discloses a waste water treatment device including a combination of a chemical softening device, an ion-exchange equipment, a reverse osmosis membrane device, and the like using the lime soda process.

CITATION LIST Patent Literature

-   {PTL 1} U.S. Pat. No. 7,815,804

SUMMARY OF INVENTION Technical Problem

The lime soda process requires the addition of sodium carbonate for the treatment, and thus the treatment cost is high. In the lime soda process, when 1 mol of Ca ions are precipitated as calcium carbonate, 2 mol of Na⁺ is produced. Meanwhile, in the case where SO₄ ²⁻ is contained in water to be treated, it is not removed by the lime soda process. That is, in the lime soda process, water after the treatment contains an increased number of moles of ions.

Also in the case where Ca ions are removed using an ion-exchange equipment, the treatment of 1 mol of Ca ions results in the production of 2 mol of Na⁺, and water the after treatment contains an increased number of moles of ions.

According to the system of Patent Literature 1, water that has been treated by the lime soda process and in an ion-exchange equipment is further treated in a reverse osmosis membrane device to remove ion components. Accordingly, the system of Patent Literature 1 has a problem in that because of the increased number of moles of ions, the osmotic pressure in the reverse osmosis membrane device is high, resulting in an increased treatment load. In addition, with the device of Patent Literature 1, SO₄ ²⁻ is not removed but remains in the treated water, and it has been difficult to obtain high water recovery.

In addition, the waste water treatment device of Patent Literature 1 requires a large amount of chemicals for the reproduction of the ion-exchange equipment, and thus there has also been the problem of high treatment cost.

An object of the present invention is to provide a water treatment process and a water treatment system, which are capable of reproducing water containing salts with high water recovery.

Solution to Problem

A first aspect of the present invention is a water treatment process including:

a scale inhibitor supplying step of supplying a calcium scale inhibitor which is a scale inhibitor for inhibiting the deposition of a scale containing calcium and a silica scale inhibitor which is a scale inhibitor for inhibiting the deposition of silica to water to be treated containing Ca ions, SO₄ ions, carbonate ions, and silica;

-   -   a demineralizing step of separating the water to be treated into         concentrated water in which the Ca ions, the SO₄ ions, the         carbonate ions, and the silica are concentrated and treated         water after the scale inhibitor supplying step; and

a crystallizing step of supplying seed crystals of gypsum to the concentrated water so that gypsum is crystallized from the concentrated water.

A second aspect of the present invention is a water treatment system including:

a scale inhibitor supplying section that supplies a calcium scale inhibitor which is a scale inhibitor for inhibiting the deposition of a scale containing calcium and a silica scale inhibitor which is a scale inhibitor for inhibiting the deposition of silica to water to be treated containing Ca ions, SO₄ ions, carbonate ions, and silica;

a demineralizing section that is installed on a downstream side of the scale inhibitor supplying section and separates the water to be treated into concentrated water in which the Ca ions, the SO₄ ions, the carbonate ions, and the silica are concentrated and treated water; and

a crystallizing section including a crystallizing tank that is installed on the downstream side of the demineralizing section and crystallizes gypsum from the concentrated water and a seed crystal supplying section that supplies seed crystals of gypsum to the crystallizing tank.

In the first aspect and the second aspect, owing to the effects of the calcium scale inhibitor and the silica scale inhibitor supplied, the production of scales in the demineralizing section and the demineralizing step can be inhibited. In addition, by adding seed crystals of gypsum to the concentrated water in the crystallizing section and the crystallizing step, even when a scale inhibitor is present, gypsum can be crystallized and separated from the water to be treated. As a result, the water to be treated containing Ca ions, SO₄ ions, the carbonate ions, and silica can be treated with high water recovery, and the operation cost can be reduced. Further, this is also advantageous in that high-purity gypsum can be recovered.

In the above aspect, the water treatment process includes a downstream side demineralizing step of separating the concentrated water after the crystallizing step on the most downstream of the water to be treated into concentrated water and treated water, and recovering the separated treated water.

In the above aspect, the water treatment system includes, on the downstream side of the crystallizing section on the most downstream of the water to be treated, a downstream side demineralizing section that separates the concentrated water discharged from the crystallizing section into concentrated water and treated water.

When the downstream side demineralizing step and the downstream side demineralizing section are provided, the water recovery can be further improved. In addition, in the invention, the number of moles of ions in the water to be treated is significantly reduced. Accordingly, the amount of salts flowing into the downstream side demineralizing section can be reduced, and thus the power of the downstream side demineralizing section can be reduced.

In the above aspect, it is preferable that the water treatment process includes a pH adjusting step of adjusting the concentrated water to a pH at which a scale inhibition function of the calcium scale inhibitor is reduced, thereby promoting the deposition of the gypsum in the crystallizing step.

In the above aspect, it is preferable that the water treatment system includes a pH adjusting section that is installed on the downstream side of the demineralizing section and supplies a pH adjuster to the concentrated water to adjust the pH of the concentrated water to such a value that a scale inhibition function of the calcium scale inhibitor is reduced, and the deposition of the gypsum is promoted.

In this case, it is preferable that after the crystallizing step, the concentrated water after the adjustment of the pH in the pH adjusting step is adjusted to a pH at which the calcium scale inhibitor exerts its function.

It is also preferable that the water treatment system includes, on the downstream side of the crystallizing section, a pH adjusting section for achieving scale inhibition function that supplies a pH adjuster to the concentrated water after the adjustment of the pH in the pH adjusting section to adjust the pH of the concentrated water to such a value that the calcium scale inhibitor achieves a function.

Alternatively, in the above aspect, the water treatment process may include a pH adjusting step of adjusting the concentrated water to a pH at which the silica is soluble in the crystallizing step. In the above aspect, it is preferable that the water treatment system includes a pH adjusting section that is installed on the downstream side of the demineralizing section and supplies a pH adjuster to the second concentrated water to adjust the pH of the concentrated water to such a value that the silica is soluble in the concentrated water in the second crystallizing section.

As a result of this configuration, the formation of scales containing calcium in the downstream side demineralizing section can be suppressed.

In the first aspect, it is preferable that the water treatment process includes a upstream side precipitating step of precipitating at least calcium carbonate from the water to be treated so that the concentration of the calcium carbonate in the water to be treated is reduced, before the scale inhibitor supplying step on the most upstream side of the water to be treated. In this case, it is preferable that the water treatment process includes a deaerating step of removing CO₂ from the water to be treated before the upstream side precipitating step or after the upstream side precipitating step and before the scale inhibitor supplying step.

In the second aspect, it is preferable that the water treatment system includes, on the upstream side of the scale inhibitor supplying section located on the most upstream of the water to be treated, a upstream side precipitating section that precipitates at least calcium carbonate from the water to be treated so that the concentration of the calcium carbonate in the water to be treated is reduced. In this case, it is preferable that the water treatment system includes a deaerating section that removes CO₂ from the water to be treated on the upstream side of the upstream side precipitating section or on the downstream side of the upstream side precipitating section and on the upstream side of the scale inhibitor supplying section.

In this way, by previously removing calcium carbonate from the water to be treated before flowing into the demineralizing section, the deposition of calcium carbonate as scales during the water treatment can be inhibited. By removing calcium carbonate, the purity of gypsum crystallized in the crystallizing step and the crystallizing section can be increased.

In the above aspect, it is preferable that the water to be treated contains metal ions, and the water treatment process includes a precipitating step of precipitating at least one of calcium carbonate and a metal compound so that the concentration of at least one of the calcium carbonate and the metal ions is reduced from the concentrated water, after the crystallizing step. In this case, at least one of seed crystals of the silica and a precipitant for the silica is supplied to the concentrated water in the precipitating step.

In the above aspect, it is preferable that the water to be treated contains metal ions, and the water treatment system includes, on the downstream side of the crystallizing section, a precipitating section that precipitates at least one of calcium carbonate and a metal compound so that the concentration of at least one of the calcium carbonate and the metal ions in the concentrated water is reduced. In this case, at least one of seed crystals of the silica and a precipitant for the silica is supplied to the precipitating section.

By removing calcium carbonate and a metal compound from the water to be treated in the precipitating section and the precipitating step provided after the crystallizing section and the crystallizing step, high water recovery can be obtained.

The dissolution state of silica changes depending on the pH of the water to be treated, but silica tends not to be deposited only by changing the pH. Thus, seed crystals of silica are added in the precipitating section and the precipitating step to promote the deposition of silica, whereby the silica removal efficiency can be improved. As a result, the water recovery can be further improved, and the operation power can be further lowered.

In the water treatment process of the above aspect, it is preferable that when the water to be treated contains Mg ions, the amount of the precipitant for silica to be supplied is adjusted according to the concentration of the Mg ions.

In the above aspect, it is preferable that when the water to be treated contains Mg ions, the amount of the precipitant for silica to be supplied is adjusted according to the concentration of the Mg ions in the precipitating section.

In the above aspect, it is preferable that when the water to be treated contains Mg ions, the concentrated water in the precipitating step is adjusted to a pH at which a magnesium compound is deposited so that the concentration of the Mg ions is reduced, and after the precipitating step, the concentrated water is adjusted to a pH at which the magnesium compound is soluble.

In the above aspect, it is preferable that when the water to be treated contains Mg ions, the concentrated water in the precipitating section is adjusted to a pH at which a magnesium compound is deposited so that the concentration of the Mg ions is reduced, and, on the downstream side of the precipitating section, the concentrated water is adjusted to a pH at which the magnesium compound is soluble.

In the case where Mg ions are contained in the water to be treated, Mg ions react with silica in the concentrated water in the precipitating step and the precipitating section, resulting in precipitation. In the present invention, the amount of precipitant to be supplied is adjusted according to the balance between Mg ions and silica in the concentrated water, whereby the precipitant is efficiently supplied. In the case where the concentration of Mg ions in high relative to silica, the pH of the concentrated water is adjusted so that a magnesium compound is deposited in the precipitating step and the precipitating section. Subsequently, the concentrated water is adjusted to a pH at which the magnesium compound is soluble, thereby suppressing the formation of scales in the demineralizing section located on the downstream side of the precipitating section.

In the above aspect, it is preferable that when the water to be treated contains Mg ions, the water to be treated in the upstream side precipitating step is adjusted to a pH at which a magnesium compound is deposited so that the concentration of the Mg ions is reduced, and, after the upstream side precipitating step, the water to be treated is adjusted to a pH at which the magnesium compound is soluble.

In the above aspect, it is preferable that when the water to be treated contains Mg ions, the water to be treated in the upstream side precipitating section is adjusted to a pH at which a magnesium compound is deposited so that the concentration of the Mg ions is reduced, and, on the downstream side of the upstream side precipitating section, the water to be treated is adjusted to a pH at which the magnesium compound is soluble.

In this way, in the case where Mg ions are contained in the water to be treated, by efficiently removing Mg ions before a demineralization treatment, the formation of scales containing magnesium in the course of water treatment can be inhibited.

In the above aspect, it is preferable that moisture is evaporated from the concentrated water in the downstream side demineralizing step, so that a solid in the concentrated water is recovered. It is preferable that the water treatment system includes, on the downstream side of the concentrated water in the downstream side demineralizing section, an evaporator that evaporates moisture from the concentrated water to recover a solid in the concentrated water.

According to the water treatment process and the water treatment system thus configured, when solid matters produced in the course of water treatment are discharged out of the system as waste, the volume of waste can be reduced.

Advantageous Effects of Invention

According to the water treatment system and the water treatment process of the present invention, while inhibiting the production of scales such as calcium carbonate and silica during the treatment, Ca²⁺ and SO₄ ²⁻ can be removed as gypsum from the water to be treated. Accordingly, the water recovery can be further improved.

Also in the case where magnesium ions are contained in the water to be treated, when the water treatment system or the water treatment process of the present invention is used, they can be removed from the water to be treated while inhibiting the production of scales containing magnesium during the treatment.

Water treated by the present invention has a significantly reduced number of moles of ions on the downstream side. Therefore, the power of the demineralizing section located downstream can be significantly reduced.

Further, the present invention is also advantageous in that high-purity gypsum can be crystallized and recovered.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of a water treatment system according to the first reference embodiment.

FIG. 2 shows simulation results for the pH dependency of the amount of gypsum deposited.

FIG. 3 shows simulation results for the pH dependency of the amount of calcium carbonate deposited.

FIG. 4 is a graph showing the pH dependency of the amount of silica dissolved.

FIG. 5 shows the results of gypsum deposition experiments performed using simulated water in which gypsum is supersaturated with changing the pH of the simulated water.

FIG. 6 shows the results of gypsum deposition experiments performed using simulated water in which gypsum is supersaturated with changing the concentration of seed crystals.

FIG. 7 is a microphotograph of gypsum crystallized under Condition 5.

FIG. 8 is a microphotograph of gypsum crystallized under Condition 3.

FIG. 9 is a schematic diagram of a water treatment system according to the second reference embodiment.

FIG. 10 is a schematic diagram of a water treatment system according to the first embodiment.

FIG. 11 is a schematic diagram of a water treatment system according to the third reference embodiment.

FIG. 12 is a schematic diagram of a water treatment system according to the fourth reference embodiment.

FIG. 13 is a schematic diagram explaining a water treatment system according to the fifth reference embodiment.

FIG. 14 is a schematic diagram explaining a water treatment system according to the sixth reference embodiment.

FIG. 15 is a schematic diagram explaining a water treatment system according to the seventh reference embodiment.

FIG. 16 is a schematic diagram explaining a water treatment system according to the eighth reference embodiment.

DESCRIPTION OF EMBODIMENTS

Water that is an object to be treated in the present invention (water to be treated) contains Ca²⁺, SO₄ ²⁻, carbonate ions, and silica. Specifically, the water to be treated (raw water) is saline water, sewage, industrial waste water, blowdown water from a cooling tower, or the like. The water to be treated may also contain metal ions, such as Mg ions.

First Reference Embodiment

FIG. 1 is a schematic diagram of a water treatment system according to the first reference embodiment of the present invention. The water treatment system 1 of FIG. 1 is configured such that two water treatment sections are connected in the flow direction of the water to be treated. In the water treatment system 1 of this reference embodiment, depending on the properties of the water to be treated, the number of water treatment sections may be one, and it is also possible that three or more water treatment sections are connected.

Each water treatment section includes, from the upstream side of the water to be treated, a first demineralizing section 10 (10 a, 10 b) and a first crystallizing section 20 (20 a, 20 b). The concentration sides of the first demineralizing sections 10 a and 10 b are connected to the first crystallizing sections 20 a and 20 b, respectively. The water treatment section includes a first scale inhibitor supplying section 30 (30 a, 30 b) and a first pH adjusting section 40 (40 a, 40 b) in the flow path on the upstream side of each first demineralizing section 10 (10 a, 10 b).

The first scale inhibitor supplying section 30 (30 a, 30 b) is made up of a tank 31 (31 a, 31 b), a valve V1 (V1 a, V1 b), and a control section 32 (32 a, 32 b). The control sections 32 a and 32 b are connected to the valves V1 a and V1 b, respectively. The tanks 31 a and 31 b have stored therein a scale inhibitor.

The scale inhibitor used in this reference embodiment serves to inhibit the deposition of scales containing calcium in the water to be treated. It will be hereinafter referred to as “calcium scale inhibitor”.

The calcium scale inhibitor suppresses the crystal nucleation of gypsum or calcium carbonate in the water to be treated. At the same time, the calcium scale inhibitor adheres to the surface of crystal nucleus of gypsum or calcium carbonate contained in the water to be treated (seed crystals, small-diameter scales deposited due to the exceeding of the saturation concentration, etc.), and functions to suppress the crystal growth of gypsum or calcium carbonate. Alternatively, there is another type of calcium scale inhibitor, which has the function of dispersing particles in the water to be treated (inhibiting aggregation), such as deposited crystals.

Examples of calcium scale inhibitors include phosphonic-acid-based scale inhibitors, polycarboxylic-acid-based scale inhibitors, and mixtures thereof. A specific example is FLOCON260 (trade name, manufactured by BWA).

In the case where Mg ions are contained in the water to be treated, a scale inhibitor that inhibits the deposition of scales containing magnesium (e.g., magnesium hydroxide) in the water to be treated can be used. It will be hereinafter referred to as “magnesium scale inhibitor”.

Examples of magnesium scale inhibitors include polycarboxylic-acid-based scale inhibitors, etc. A specific example is FLOCON 295N (trade name, manufactured by BWA).

Although FIG. 1 shows only one first scale inhibitor supplying section 30 a/30 b in each position, in the case where two or more kinds of scale inhibitors are loaded, it is preferable that two or more first scale inhibitor supplying sections are installed. In this case, the scale inhibitors are sorted according to kind and stored in the respective tanks.

The first pH adjusting section 40 (40 a, 40 b) is made up of a tank 41 (41 a, 41 b), a valve V2 (V2 a, V2 b), a control section 42 (42 a, 42 b), and a pH meter 43 (43 a, 43 b). The tanks 41 a and 41 b have stored therein an alkali as a pH adjuster. The alkali is calcium hydroxide or sodium hydroxide, for example. Calcium hydroxide is particularly preferable because Ca ions are recovered as gypsum in the below-mentioned crystallizing step, and thus the amount of ions that reach the demineralizing section on the downstream side is reduced. The control sections 42 a and 42 b are connected to the valves V2 a and V2 b and the pH meters 43 a and 43 b, respectively.

In FIG. 1, the first demineralizing sections 10 a and 10 b are reverse osmosis membrane devices. In addition, the first demineralizing sections 10 a and 10 b may also be electrodialyzers (ED), electro dialysis reversal devices (EDR), electro de-ionization devices (EDI), ion-exchange equipments (IEx), capacitive de-ionization devices (CDI), nanofilters (NF), evaporators, etc.

Here, in a nanofilter (NF), an electrodialyzer (ED), an electro dialysis reversal device (EDR), an electro de-ionization device (EDI), and a capacitive de-ionization device (CDI), scale components (divalent ions, Ca²⁺, Mg²⁺, etc.) are selectively removed, while monovalent ions such as Na⁺ and Cl⁻ permeate. The use of these demineralizers suppresses an increase in the ion concentration of ions that serve as scale components in concentrated water. Accordingly, the water recovery can be improved, and also energy saving (e.g., the reduction of pump power, etc.) can be achieved.

In addition, in the case where the water to be treated is blowdown water from a cooling tower, the reclaimed water does not have to be pure water, and what is necessary is that scale components (divalent ions, Ca²⁺, Mg²⁺, etc.) are removed. Accordingly, it is advantageous to use a nanofilter (NF), etc.

Although only one first demineralizing section 10 a/10 b is shown in FIG. 1, the system may also be configured such that two or more demineralizers are connected in parallel or in series in the flow direction of the water to be treated.

The first crystallizing section 20 (20 a, 20 b) is made up of a first crystallizing tank 21 (21 a, 21 b) and a first seed crystal supplying section 22 (22 a, 22 b). The first seed crystal supplying sections 22 a and 22 b are connected to the first crystallizing tanks 21 a and 21 b, respectively. The first seed crystal supplying sections 22 a and 22 b have a seed crystal tank 23 (23 a, 23 b), a valve V3 (V3 a, V3 b), and a control section 24 (24 a, 24 b). The control sections 24 a and 24 b are connected to the valves V3 a and V3 b, respectively. The seed crystal tanks 23 a and 23 b store gypsum particles as seed crystals.

In the water treatment system 1 of FIG. 1, a first precipitating section 50 (50 a, 50 b) may be installed on the downstream side of each of the first crystallizing sections 20 a and 20 b. The first precipitating sections 50 a and 50 b each include a first precipitating tank 51 (51 a, 51 b) and a first filtration device 52 (52 a, 52 b).

The water treatment system 1 includes a downstream side demineralizing section 60 on the downstream side of the water to be treated of the first crystallizing section 20 b located on the most downstream.

In FIG. 1, the downstream side demineralizing section 60 is a reverse osmosis membrane device. The downstream side demineralizing section 60 may also be an electrodialyzer (ED), an electro dialysis reversal device (EDR), an electro de-ionization device (EDI), an ion-exchange equipment, a capacitive de-ionization device (CDI), a nanofilter (NF), an evaporator, etc.

In the water treatment system 1, a precipitating tank 71 and a filtration device 72 are installed as a first upstream side precipitating section 70 on the upstream side of the first scale inhibitor supplying section 30 a and the first pH adjusting section 40 a which are located on the most upstream of the water to be treated. The precipitating tank 71 and the filtration device 72 have the same configuration as the first precipitating tank 51 and the first filtration device 52 of the first precipitating section 50.

In particular, in the case where Mg ions are contained in the water to be treated, the first upstream side precipitating section can be configured such that two or more precipitating tanks 71 are connected in series in the flow direction of the water to be treated.

In the water treatment system 1 shown in FIG. 1, a first deaerating section 73 may be provided on the upstream side of the first upstream side precipitating section 70. Specifically, the first deaerating section 73 is a deaeration tower equipped with a filler for removing carbon dioxide or is a separation membrane. On the upstream side of the water to be treated of the first deaerating section 73, a pH adjusting section for carbonate ions (not shown) that adjusts the water to be treated to a pH at which carbonate ions are present in the form of CO₂ may be installed.

The first deaerating section 73 may also be installed on the downstream side of the water to be treated of the first upstream side precipitating section 70 and on the upstream side of the first scale inhibitor supplying section 30 a and the first pH adjusting section 40 a.

It is also possible that a deaerating section having the same configuration as the first deaerating section 73 is installed in the flow path between the first demineralizing section 10 and the first crystallizing section 20, in the flow path between the first crystallizing section 20 and the first precipitating section 50, and on the downstream side of the first precipitating section 50 and in the flow path between it and the first demineralizing section 10 b or the downstream side demineralizing section 60.

In the case where the concentration of Ca ions in the water to be treated is high, an ion-exchange equipment (not shown) may be installed on the downstream of the filtration device 72 and on the upstream of the first scale inhibitor supplying section 30 a and the first pH adjusting section 40 a which are located on the most upstream. The ion-exchange equipment may be an ion-exchange resin column or an ion-exchange membrane device, for example.

When gypsum in the water to be treated flowing into the first demineralizing section 10 a is already supersaturated, because ions are further concentrated in the first demineralizing section 10 a, the resulting gypsum concentration is even higher. In this case, the loading of a large amount of calcium scale inhibitor is required. Further, the concentration of gypsum may become too high for the calcium scale inhibitor to exert its effect, resulting in the production of scales in the first demineralizing section 10 a.

Thus, in the case where gypsum in the raw water (water to be treated) is supersaturated, it is possible that an upstream side crystallizing section (not shown) having the same configuration as the first crystallizing tanks 21 a and 21 b are provided on the upstream of the first scale inhibitor supplying section 30 a and the first pH adjusting section 40 a on the most upstream, so that the concentration of gypsum is reduced, and then the water to be treated is fed to the first demineralizing section 10 a.

A process for treating water to be treated using the water treatment system 1 of the first reference embodiment will be described hereinafter.

First, the deposition behaviors of gypsum, silica, and calcium carbonate in water will be explained. FIG. 2 shows simulation results for the pH dependency of the amount of gypsum deposited. FIG. 3 shows simulation results for the pH dependency of the amount of calcium carbonate deposited. In the figures, the abscissa is pH, and the ordinate is the amount of gypsum or calcium carbonate deposited (mol). Using a simulation software manufactured by OLI, the simulation was performed under the conditions where 0.1 mol/L of each solid component was mixed with water, and H₂SO₄ and Ca(OH)₂ were added as an acid and an alkali, respectively.

FIG. 4 is a graph showing the pH dependency of the amount of silica dissolved (source: FIG. 4 of U.S. Pat. No. 7,815,804). In the figure, the abscissa is pH, and the ordinate is the amount of silica dissolved (mg/L).

From FIG. 2, it can be understood that gypsum deposition has no pH dependency, and deposition is possible over the entire pH range. However, when a calcium scale inhibitor is added, in a high-pH region, gypsum is present in the state of being dissolved in water. From FIG. 3, calcium carbonate is deposited when the pH is more than 5. From FIG. 4, silica tends to dissolve in water when the pH is 10 or more.

<Pretreatment>

In the case where the water to be treated is industrial waste water, etc., before the water to be treated flows into the first upstream side precipitating section 70, a step of removing oils, floating particles, and the like from the water to be treated and a step of removing organic substances by a biological treatment or a chemical oxidation treatment are performed.

<First Deaerating Step>

In the water treatment system 1 of FIG. 1, the water to be treated before flowing into the first deaerating section 73 is adjusted to a low pH. Carbonic acid in the water to be treated is in the following equilibrium depending on the pH of the water to be treated. {Chemical Formula 1} CO₂

H₂CO₃

HCO₃ ⁻+H⁺

CO₃ ²⁻+2H⁺  (1)

In the case where the pH is as low as 6.5 or less, it is mainly present as HCO₃ ⁻ and CO₂ in the water to be treated.

The water to be treated containing CO₂ flows into the first deaerating section 73. CO₂ is removed from the water to be treated in the first deaerating section 73. When the water to be treated has been previously adjusted to a pH at which carbonate ions are present as CO₂, carbon dioxide can be efficiently removed.

The water to be treated, whose carbonate ion concentration has been reduced in the first deaerating step, is fed to the first upstream side precipitating section 70.

<First Upstream Side Precipitating Step>

In the first upstream side precipitating section 70, some of Ca ions and carbonate ions are previously removed from the water to be treated as calcium carbonate.

In the case where metal ions other than Ca ions are contained in the water to be treated, in the first upstream side precipitating section 70, some of the metal ions are previously removed from the water to be treated as a metal compound having low solubility in water. This metal compound is mainly a metal hydroxide, but may also include a carbonate.

In the precipitating tank 71, Ca(OH)₂ and an anionic polymer (manufactured by Mitsubishi Heavy Industries Mechatronics Systems, Ltd., trade name: Hishifloc H305) are loaded to the water to be treated, and the pH in the precipitating tank 71 is controlled to 4 or more and 12 or less, and preferably 8.5 or more and 12 or less.

As shown in FIG. 3, the solubility of calcium carbonate is low in this pH range. When calcium carbonate is supersaturated, calcium carbonate is deposited and precipitated at the bottom of the precipitating tank 71.

The solubility of a metal compound depends on pH. A more acidic pH leads to a higher solubility of metal ions in water. For many metal compounds, the solubility is low in the above pH range. In the above pH range, a metal compound having low solubility in water aggregates in the precipitating tank 71, resulting in precipitation at the bottom of the precipitating tank 71.

The precipitated calcium carbonate and metal compound are discharged from the bottom of the precipitating tank 71.

Mg ions form salts that are poorly soluble in water, and thus are components that tend to be deposited as scales. Mg(OH)₂ is deposited at pH 10 or more.

In the case where the water to be treated containing Mg ions is treated by the water treatment system 1 of this embodiment, the pH of the water to be treated in the precipitating tank 71 is adjusted to a pH at which a magnesium compound (mainly magnesium hydroxide) is deposited. Specifically, the pH of the water to be treated is adjusted to 10 or more, preferably 10.5 or more, and more preferably 11 or more. Accordingly, a magnesium compound is deposited from the water to be treated, precipitated at the bottom of the precipitating tank 71, and removed. As a result, some of Mg ions in the water to be treated are removed, resulting in a decrease in the concentration of Mg ions in the water to be treated.

In the above case, it is preferable that the water to be treated after being discharged from the first upstream side precipitating section 70 is adjusted to a pH at which the above magnesium compound is soluble. Specifically, the pH is adjusted to less than 10. Accordingly, the formation of scales in devices and steps on the downstream side, particularly the first demineralizing section 10 a and the first demineralizing step, can be inhibited.

In the case where two or more stages of precipitating tanks 71 are provided, Mg ions in the water to be treated can be reliably removed, and the concentration of Mg ions in the water to be treated fed to the downstream side can be reduced.

The supernatant in the precipitating tank 71, which is the water to be treated, is discharged from the precipitating tank 71. FeCl₃ is added to the discharged water to be treated, and solids in the supernatant, such as calcium carbonate and a metal compound, aggregate with Fe(OH)₃.

The water to be treated is fed to the filtration device 72. The solids aggregated with Fe(OH)₃ are removed through the filtration device 72.

In the case where the first deaerating step is performed after the first upstream side precipitating step, the pH of the water to be treated is adjusted to a pH at which carbonate ions can be present as CO₂, specifically 6.5 or less.

Incidentally, depending on the properties of the water to be treated, the first deaerating step and the first upstream side precipitating step may be omitted.

In the case where an ion-exchange equipment is installed, Ca ions in the water to be treated are removed by the ion-exchange equipment. In the case where Mg ions are contained in the water to be treated, the Mg ions are also removed by the ion-exchange equipment.

In the case where gypsum in the raw water is supersaturated, seed crystals of gypsum are loaded to the water to be treated in the upstream side crystallizing section installed immediately after the filtration device 72, and gypsum is crystallized, thereby reducing the concentration of gypsum in the water to be treated. The water to be treated having a reduced concentration of gypsum is fed to the first demineralizing section 10 a.

<First Scale Inhibitor Supplying Step>

The control section 32 a of the first scale inhibitor supplying section 30 a opens the valve Via and supplies a predetermined amount of calcium scale inhibitor to the water to be treated from the tank 31 a. The control section 32 a adjusts the opening of the valve Via so that the concentration of the calcium scale inhibitor is a predetermined value set according to the properties of the water to be treated.

In the case where Mg ions are contained in the water to be treated, a magnesium scale inhibitor is supplied to the water to be treated in the first scale inhibitor supplying step in the same manner as above. In this case, the calcium scale inhibitor and the magnesium scale inhibitor are stored in the tank of each of two or more first scale inhibitor supplying sections, and each control section adjusts the amounts of calcium scale inhibitor and magnesium scale inhibitor to be supplied.

<First pH Adjusting Step>

The control section 42 a of the first pH adjusting section 40 a controls the pH of the water to be treated at the entrance of the first demineralizing section 10 a to such a value that silica is soluble in the water to be treated. Specifically, the pH of the water to be treated fed to the first demineralizing section 10 a is adjusted to 10 or more, preferably 10.5 or more, and more preferably 11 or more.

The pH meter 43 a measures the pH of the water to be treated at the entrance of the first demineralizing section 10 a. The control section 42 a adjusts the opening of the valve V2 a so that the value measured by the pH meter 43 a is a predetermined pH control value, and allows an alkali to be loaded to the water to be treated from the tank 41 a.

<First Demineralizing Step>

In the first demineralizing section 10 a, the pH-adjusted water to be treated is treated. In the case where the first demineralizing section 10 a is a reverse osmosis membrane device, the water that has passed through the reverse osmotic membrane is recovered as treated water. Ions and scale inhibitors contained in the water to be treated cannot pass through the reverse osmosis membrane. Therefore, on the non-permeate side of the reverse osmosis membrane, there is concentrated water having a high concentration of ions. Also in the case where other demineralizers, such as a capacitive de-ionization device, are used, for example, the water to be treated is separated into treated water and concentrated water having a high concentration of ions (first concentrated water).

As shown in FIG. 4, as a result of the first demineralizing step, silica is contained in the first concentrated water in the state of being dissolved in the water to be treated. Even in the case where gypsum and calcium carbonate in the first concentrated water are concentrated to the saturation concentration or higher, the production of scales is suppressed by the calcium scale inhibitor.

In the case where Mg ions are contained in the water to be treated, the concentration of Mg ions contained in the first concentrated water increases as a result of the first demineralizing step. However, the production of scales containing magnesium is suppressed by the magnesium scale inhibitor.

The first concentrated water is fed toward the first crystallizing section 20 a.

<First Crystallizing Step>

The first concentrated water discharged from the first demineralizing section 10 a is stored in the first crystallizing tank 21 a of the first crystallizing section 20 a. The control section 24 a of the first seed crystal supplying section 22 a opens the valve V3 a and adds seed crystals of gypsum to the first concentrated water in the first crystallizing tank 21 a from the seed crystal tank 23 a.

The pH of the first concentrated water from the first demineralizing section 10 a is 10 or more. As mentioned above, gypsum is in the state of being dissolved in water in a high-pH region where a calcium scale inhibitor is present. However, when seed crystals are sufficiently present, even when a scale inhibitor is present, gypsum is crystallized using the seed crystals as nuclei. In the water treatment system 1 of FIG. 1, the crystal-grown gypsum having a large diameter (e.g., having a particle diameter of 10 μm or more, more preferably 20 μm or more) is precipitated at the bottom of the first crystallizing tank 21 a. The precipitated gypsum is discharged from the bottom of the first crystallizing tank 21 a.

Meanwhile, when the pH 10 is or more, silica is present in the state of being dissolved in the first concentrated water in the first crystallizing tank 21 a. Even in the case where the concentration of silica in the first concentrated water exceeds the saturation solubility, because seed crystals of silica are not present, silica is deposited as floating matters in a colloidal form or the like and unlikely to be precipitated.

With reference to FIG. 3, calcium carbonate tends to be deposited at pH 10 or more. However, because the calcium scale inhibitor has been added, the deposition of calcium carbonate is suppressed in the first crystallizing tank 21 a. In addition, in the case where the first upstream side precipitating section or the first deaerating section is provided, the concentration of calcium carbonate has been previously reduced. As a result, in the first crystallizing tank 21 a, calcium carbonate is unlikely to be crystallized using the seed crystals of gypsum as nuclei.

Incidentally, although gypsum is crystallized independent of pH when seed crystals of gypsum are present, the crystallization rate increases with a decrease in pH.

FIG. 5 shows the result of gypsum deposition experiments with changing the pH of simulated water in the case where a scale inhibitor (FLOCON260) is added to simulated water (containing Ca²⁺, SO₄ ²⁻, Na⁺, and Cl⁻) in which gypsum is supersaturated. The experimental conditions are as follows:

The degree of gypsum supersaturation in simulated water (25° C.): 460%,

The amount of scale inhibitor to be added: 2.1 mg/L,

pH: 6.5 (Condition 1), 5.5 (Condition 2), 4.0 (Condition 3), 3.0 (Condition 4),

The amount of seed crystals to be added: 0 g/L.

Two hours and 6 hours immediately after the pH adjustment, the concentration of Ca in the simulated water treated under each condition was measured using an atomic absorption spectrometer (manufactured by Shimadzu Corporation, AA-7000), and the degree of supersaturation was calculated. The results are shown in FIG. 5. In the figure, the ordinate is the degree of supersaturation (%).

With reference to FIG. 5, even under conditions where seed crystals are absent, the crystallization rate increases with a decrease in pH. From this, it can be understood that in the case where seed crystals are present, gypsum is crystallized even under Condition 1 (pH 6.5), and the relation of the crystallization rate is such that the crystallization rate increases with a decrease in pH as shown in FIG. 5.

In the case where carbonate ions are contained in the water to be treated, under low-pH conditions, carbonate ions are removed from the water to be treated as CO₂ as in chemical formula (1). In addition, as can be understood from FIG. 3, in the case where the pH is low, calcium carbonate is in a dissolved state.

From these results, when the first crystallizing step is performed under low-pH conditions, because of the low content of calcium carbonate and silica, high-purity gypsum is crystallized and recovered from the bottom of the first crystallizing tank 21 a. In the case where the first crystallizing step is performed at low pH, a third pH adjusting section (a pH adjusting section for achieving the calcium scale inhibition function, not shown) that supplies an acid as a pH adjuster is installed in the first crystallizing tank 21 a or in the flow path between the first demineralizing section 10 a and the first crystallizing tank 21 a. The pH adjusting section has the same configuration as the below-mentioned second pH adjusting section.

Meanwhile, in order to change the pH in the course of water treatment, it is necessary to supply a large amount of chemicals (acid or alkali). The use of an acid or an alkali leads to an increase in the amount of ions transferred to the downstream side of the first crystallizing section 20 a, and this causes an increase in the power of demineralizing sections on the downstream side (in FIG. 1, the first demineralizing section 10 b or the downstream side demineralizing section 60). In terms of operation cost, it is more advantageous that the pH is not changed between the first demineralizing step and the first crystallizing step.

The gypsum crystallization rate depends on the loading of seed crystals. FIG. 6 shows the results of gypsum deposition experiments with changing the amount of seed crystals to be added in the case where a calcium scale inhibitor (FLOCON260) is added to simulated water. The experimental conditions were the same as in FIG. 5 except that the pH was 4.0, and that gypsum (CaSO₄-2H₂O) was added as seed crystals in the following amounts:

The amount of seed crystals to be added: 0 g/L (Condition 3), 3 g/L (Condition 5), 6 g/L (Condition 6), 3 g/L (Condition 7).

Under Conditions 5 and 6, seed crystals and sulfuric acid for pH adjustment were added to the simulated water having added thereto a scale inhibitor. Under Condition 7, seed crystals pre-immersed in the above scale inhibitor were added to the simulated water having added thereto a scale inhibitor, and sulfuric acid was added for pH adjustment.

Two hours immediately after the pH adjustment, the concentration of Ca in the simulated water treated under each condition was measured by the same technique as in FIG. 5. In FIG. 6, the ordinate is the degree of supersaturation (%).

From the results of FIG. 6, it can be understood that although the degree of supersaturation was 215% under Condition 3 where seed crystals are not added, the degree of supersaturation decreases to 199% (Condition 5) and 176% (Condition 6) with an increase in the concentration of seed crystals, leading to an increase in the gypsum deposition rate. Also under high-pH conditions, similarly, the gypsum deposition rate tends to increase with an increase in the loading of seed crystals. Condition 5 and Condition 7 are the same test conditions, except for whether the used seed crystals are not immersed or immersed in a scale inhibitor. Also under Condition 7 where seed crystals have a scale inhibitor previously adhering thereto, the degree of supersaturation is 199%, and it has been confirmed that gypsum is deposited at the same level as under Condition 5. That is, the results under Condition 5 and 7 show that independent of the immersion time of seed crystals in a calcium scale inhibitor, when the pH is reduced to 4.0, the function of the scale inhibitor is reduced.

FIGS. 7 and 8 each show a microphotograph of gypsum resulting from crystallization. FIG. 7 shows results under Condition 5 (seed crystals added), and FIG. 8 shows results under Condition 3 (no seed crystals added). Under Condition 5, gypsum having a larger size was deposited than under Condition 3. Generally, the water content decreases with an increase in the size of deposited gypsum. A low water content leads to high-purity gypsum. When the average particle diameter is 10 μm or more, preferably 20 μm or more, the resulting gypsum has a sufficiently reduced water content. The “average particle diameter” in the present invention is a particle diameter measured by the method specified in JIS Z 8825 (laser diffractometry).

<First Precipitating Step>

The supernatant (first concentrated water) in the first crystallizing section 20 a is fed to the first precipitating section 50 a. In the first precipitating section 50 a, Ca(OH)₂ and an anionic polymer (Hishifloc H305) are loaded to the first concentrated water after the crystallizing step, and the pH in the first precipitating tank 51 a is controlled to 4 or more and 12 or less, and preferably 8.5 or more and 12 or less. In the first precipitating tank 51 a, calcium carbonate and a metal compound are precipitated and removed from the first concentrated water. The precipitated calcium carbonate and metal compound having low solubility in water are discharged from the bottom of the first precipitating tank 51 a.

The water to be treated, which is the supernatant in the first precipitating tank 51 a, is discharged from the first precipitating tank 51 a. FeCl₃ is added to the discharged water to be treated, and solids in the water to be treated, such as calcium carbonate and a metal compound, aggregate with Fe(OH)₃.

The water to be treated is fed to the first filtration device 52 a. The solids aggregated with Fe(OH)₃ are removed through the first filtration device 52 a.

Silica in the supernatant in the first crystallizing section 20 a may be removed from the first concentrated water in the first precipitating step, or may also be fed to the downstream side without being removed.

Whether silica is removed in the first precipitating step is determined according to the properties of the water to be treated or the first concentrated water.

In the case where silica is not removed, the first precipitating step is performed without supplying seed crystals of silica and a precipitant for silica to the first precipitating tank 51 a. In this case, silica is separated from the treated water in demineralizing sections located on the downstream side (the first demineralizing section 10 b and the downstream side demineralizing section 60).

In the case where silica is removed, at least one of seed crystals of silica and a precipitant for silica is supplied into the first concentrated water in the first precipitating section 50 a from a supply section (not shown). The seed crystals of silica are a silica gel, for example, and the precipitant for silica is MgSO₄ or Na aluminate (Na[Al(OH)₄]), for example. In the case where silica is removed, it is preferable that the first concentrated water in the first precipitating tank 51 a is adjusted to pH 8 or more and 10 or less. In the case where seed crystals of silica are used, silica is crystallized using the seed crystals as nuclei. In the case where MgSO₄ is used as a precipitant for silica, magnesium silicate is deposited. The crystallized silica or the crystallized magnesium silicate is precipitated at the bottom of the first precipitating tank 51 a and discharged from the bottom of the first precipitating tank 51 a.

In the case where Mg ions are contained in the water to be treated, Mg ions react with silica in the first concentrated water in the first precipitating step, resulting in precipitation. The steps for silica/Mg ion removal vary depending on the balance between the content of Mg ions and the content of silica in the first concentrated water in the first precipitating tank 51 a.

In the case where the first concentrated water in the first precipitating step has a lower concentration of Mg ions relative to the silica content, Mg ions are consumed by precipitation with silica. In order to remove an excess of silica that is not consumed by precipitation with Mg ions, a precipitant for silica (MgSO₄) is supplied. With respect to the amount of precipitant for silica to be supplied, according to the content of silica and the content of Mg ions in the first precipitating step, the precipitant is supplied in such an amount that the excess of silica is consumed.

In the case where the first concentrated water in the first precipitating step has a higher concentration of Mg ions relative to the silica content, Mg ions remain as a result of the precipitation of Mg ions and silica. When the first concentrated water having a high concentration of residual Mg ions is discharged from the first precipitating tank 51 a, scales containing Mg may be deposited in demineralizing sections of subsequent stages (the first demineralizing section 10 b in FIG. 1; in the case of the first precipitating section on the most downstream, the downstream side demineralizing section 60).

Thus, the first concentrated water in the first precipitating tank 51 a is adjusted to such a value that a magnesium compound (mainly magnesium hydroxide) can be deposited. Accordingly, a magnesium compound is precipitated in the first precipitating tank 51 a, thereby reducing the concentration of Mg ions in the first concentrated water in the first precipitating tank 51 a. Further, after the first precipitating step, the first concentrated water discharged from the first precipitating tank 51 a is adjusted to a pH at which the magnesium compound is soluble, specifically to a pH of less than 10. Accordingly, the deposition of scales containing Mg in a demineralizing section can be suppressed.

In the case where the treatment is performed in several stages, the first concentrated water that has passed through the first filtration device 52 a of the first water treatment section of the previous stage flows into the water treatment section of the subsequent stage as water to be treated. In the water treatment section of the subsequent stage, the steps from the first scale inhibitor supplying step to the first precipitating step mentioned above are performed.

<Downstream Side Demineralizing Step>

The concentrated water (first concentrated water) that has passed through the first precipitating section 50 b located on the most downstream of the water to be treated is fed to the downstream side demineralizing section 60. The water that has passed through the downstream side demineralizing section 60 is recovered as treated water. The concentrated water in the downstream side demineralizing section 60 is discharged out of the system. The installation of the downstream side demineralizing section 60 makes it possible to further recover treated water from water that has been treated in a water treatment section. Accordingly, the water recovery is improved.

In the water treatment system 1 of this reference embodiment, ions are concentrated in the first demineralizing section 10. However, gypsum, calcium carbonate, silica, etc., have been removed in the first crystallizing section, the first precipitating section, etc. Accordingly, the water flowing into the downstream side demineralizing section 60 has a smaller number of moles of ions than before the treatment. Accordingly, the osmotic pressure is low in the first demineralizing section 10 b or the downstream side demineralizing section 60 located downstream, and the required power is reduced.

An evaporator (not shown in FIG. 1) may be installed on the downstream on the concentrated-water side of the downstream side demineralizing section 60. In the evaporator, water is evaporated from the concentrated water, and ions contained in the concentrated water are deposited as a solid and recovered as a solid. Because water is recovered on the upstream side of the evaporator, and the amount of concentrated water significantly decreases, the evaporator can be reduced in size, and the energy required for evaporation can be reduced.

Second Reference Embodiment

FIG. 9 is a schematic diagram of a water treatment system of the second reference embodiment of the present invention. In FIG. 9, the same configurations as in the first reference embodiment are indicated with the same reference numerals. In the water treatment system 100 of the second reference embodiment, a first separating section 180 (180 a, 180 b) is installed on the downstream side of the first crystallizing sections 20 a and 20 b. The water treatment system 100 of FIG. 9 is configured such that two water treatment sections are connected in the flow direction of the water to be treated. In the water treatment system 100 of this reference embodiment, depending on the properties of the water to be treated, the number of water treatment sections may be one, and it is also possible that three or more water treatment sections are connected.

In FIG. 9, the first separating section 180 (180 a, 180 b) includes a classifier 181 (181 a, 181 b) and a dehydrator 182 (182 a, 182 b). The classifiers 181 a and 181 b are liquid cyclones, for example. The dehydrators 182 a and 182 b are belt filters, for example.

Although the first separating section 180 has only one classifier installed in FIG. 9, it is also possible that two or more classifiers are connected in series in the flow direction of the water to be treated.

In the water treatment system 100 of the second reference embodiment, the water to be treated is treated through the same steps as in the first reference embodiment, except that the first separating step is performed immediately after the first crystallizing step.

<First Separating Step>

First concentrated water in the first crystallizing tanks 21 a and 21 b is transferred to the first separating sections 180 a and 180 b. The first concentrated water transferred here is water containing solid matters deposited in the first crystallizing tanks 21 a and 21 b.

The first concentrated water discharged from the first crystallizing tanks 21 a and 21 b contains gypsum having various particle diameters, as well as calcium carbonate and silica deposited due to the exceeding of the saturation concentration. Because the deposition of calcium carbonate and silica has taken place in the absence of seed crystals, they have small diameters or are floating matters in a colloidal form.

When the first concentrated water flows into the classifiers 181 a and 181 b, gypsum having a predetermined size, for example, gypsum having an average particle diameter of 10 μm or more, sediments at the bottom of the classifiers 181 a and 181 b, and gypsum having a small particle diameter, calcium carbonate, and silica remain in the supernatant. The gypsum sedimented at the bottom of the classifiers 181 a and 181 b is further dehydrated by the dehydrators 182 a and 182 b and recovered. The supernatant containing gypsum having a small particle diameter, calcium carbonate, and silica is fed to the first precipitating sections 50 a and 50 b.

In this reference embodiment, seed crystals are added to cause crystallization. Therefore, gypsum having an average particle diameter of 10 μm or more is mainly deposited, and the proportion of gypsum having a small diameter is low. Through the first separating step, gypsum having a low water content and containing no impurities (i.e., high-purity gypsum) can be separated and recovered with high recovery.

Some of the gypsum recovered in the first separating sections 180 a and 180 b may be circulated through the seed crystal tanks 23 a and 23 b as seed crystals.

First Embodiment

FIG. 10 is a schematic diagram of a water treatment system of the first embodiment of the present invention. The water treatment system 200 of FIG. 10 is configured such that two water treatment sections are connected in the flow direction of the water to be treated. Depending on the properties of the water to be treated, the number of water treatment sections may be one, and it is also possible that three or more water treatment sections are connected.

In the water treatment system 200 of the first embodiment, each water treatment section includes, from the upstream side of the water to be treated, a second demineralizing section 210 (210 a, 210 b) and a second crystallizing section 220 (220 a, 220 b). The concentration sides of the second demineralizing sections 210 a and 210 b are connected to the second crystallizing sections 220 a and 220 b, respectively. The water treatment section includes a second scale inhibitor supplying section 230 (230 a, 230 b) in the flow path on the upstream side of each second demineralizing section 210 (210 a, 210 b).

The second scale inhibitor supplying sections 230 a and 230 b are each made up of a tank 231 (231 a, 231 b), a valve V4 (V4 a, V4 b), and a control section 232 (232 a, 232 b). The control sections 232 a and 232 b are connected to the valves V4 a and V4 b, respectively. The tanks 231 a and 231 b of the second scale inhibitor supplying sections 230 a and 230 b have stored therein a scale inhibitor.

Scale inhibitors used in the first embodiment are the calcium scale inhibitor described in the first reference embodiment and a scale inhibitor that inhibits the deposition of silica as scales in the water to be treated (referred to as “silica scale inhibitor”). Examples of silica scale inhibitors include phosphonic-acid-based scale inhibitors, polycarboxylic-acid-based scale inhibitors, and mixtures thereof. A specific example is FLOCON260 (trade name, manufactured by BWA).

FIG. 10 shows two tanks 231 a. For example, a calcium scale inhibitor is stored in one tank 231 a, and a silica scale inhibitor is stored in the other tank 231 a.

In FIG. 10, the second demineralizing section 210 is a reverse osmosis membrane device. In addition, the second demineralizing section 210 may also be an electrodialyzer (ED), an electro dialysis reversal device (EDR), an electro de-ionization device (EDI), an ion-exchange equipment, a capacitive de-ionization device (CDI), a nanofilters (NF), an evaporator, etc.

Although only one second demineralizing section 210 is shown in FIG. 10, the system may also be configured such that two or more demineralizers are connected in parallel or in series in the flow direction of the water to be treated.

The second crystallizing section 220 (220 a, 220 b) is made up of a second crystallizing tank 221 (221 a, 221 b) and a second seed crystal supplying section 222 (222 a, 222 b). The second seed crystal supplying section 222 is connected to the second crystallizing tank 221. The second seed crystal supplying section 222 has a seed crystal tank 223 (223 a, 223 b), a valve V5 (V5 a, V5 b), and a control section 224 (224 a, 224 b). The control section 224 is connected to the valve V5. The seed crystal tank 223 stores gypsum particles as seed crystals.

In the water treatment system 200 of the first embodiment, a second pH adjusting section 240 (240 a, 240 b) may be installed between the second demineralizing section 210 and the second crystallizing section 220. The second pH adjusting section 240 is made up of a tank 241 (241 a, 241 b), a valve V6 (V6 a, V6 b), a pH meter 243 (243 a, 243 b), and a control section 242 (242 a, 242 b). The tank 241 has stored therein an acid as a pH adjuster. The acid used may be hydrochloric acid, sulfuric acid, nitric acid, or the like, for example. Sulfuric acid is particularly preferable because SO₄ ²⁻ is removed as gypsum in the crystallizing step, and thus the amount of ions that reach the demineralizing section on the downstream side can be reduced. The control section 242 is connected to the valve V6 and the pH meter 243. The pH meter 243 may be installed in the flow path between the second demineralizing section 210 and the second crystallizing section 220 as shown in FIG. 10, or may also be installed in the second crystallizing tank 221.

In the water treatment system 200, a precipitating tank 271 and a filtration device 272 are installed as a second upstream side precipitating section 270 on the upstream side of the second scale inhibitor supplying section 230 a located on the most upstream of the water to be treated. The second upstream side precipitating section 270 has the same configuration as the first upstream side precipitating section 70. As in the first reference embodiment, two or more stages of precipitating tanks 271 may be connected in series in the flow direction of the water to be treated.

In the water treatment system 200, a second deaerating section 273 may be provided on the upstream side of the second upstream side precipitating section 270 as shown in FIG. 10. The second deaerating section 273 has the same configuration as the first deaerating section 73 of the first reference embodiment.

The second deaerating section 273 may be installed on the downstream side of the water to be treated of the second upstream side precipitating section 270 and on the upstream side of the second scale inhibitor supplying section 230 a.

It is also possible that a deaerating section having the same configuration as the second deaerating section 273 is installed in the flow path between the second demineralizing section 210 a and the second crystallizing section 220 a, in the flow path between the second crystallizing section 220 and the second precipitating section 250, and on the downstream side of the second precipitating section 250 and in the flow path between it and the second demineralizing section 210 b or the downstream side demineralizing section 60.

As in the first reference embodiment, an ion-exchange equipment (not shown) may be installed on the downstream of the filtration device 272 and on the upstream of the second scale inhibitor supplying section 230 a located on the most upstream. In addition, depending on the concentration of gypsum in the water to be treated, an upstream side crystallizing section (not shown) having the same configuration as the second crystallizing section may be installed on the upstream of the second scale inhibitor supplying section 230 a on the most upstream.

In this embodiment, a second separating section 280 (280 a, 280 b) may be installed on the downstream side of the second crystallizing section 220 as shown in FIG. 10. The second separating section 280 has the same configuration as the first separating section 180 and includes a classifier 281 (281 a, 281 b) and a dehydrator 282 (282 a, 282 b).

In the water treatment system 200 of FIG. 10, a second precipitating section 250 (250 a, 250 b) may be installed on the downstream side of the second crystallizing section 220. The second precipitating section 250 has the same configuration as the first precipitating section 50 and includes a second precipitating tank 251 (251 a, 251 b) and a second filtration device 252 (252 a, 252 b).

The water treatment system 200 includes a downstream side demineralizing section 60 on the downstream side of the water to be treated of the first water treatment section. An evaporator (not shown in FIG. 10) may be installed on the downstream on the concentrated-water side of the downstream side demineralizing section 60.

A process for treating water to be treated using the water treatment system 200 of the first embodiment will be described hereinafter.

<Pretreatment>

The water to be treated is subjected to the pretreatment described in the first reference embodiment.

<Second Deaerating Step>

In the same manner as in the first deaerating step described in the first reference embodiment, CO₂ in the water to be treated is removed in the second deaerating section 273, whereby the concentration of carbonate ions in the water to be treated is reduced.

<Second Upstream Side Precipitating Step>

In the second upstream side precipitating section 270, some of Ca ions and carbonate ions are previously removed from the water to be treated as calcium carbonate. In the case where metal ions other than Ca ions are contained in the water to be treated, in the second upstream side precipitating section 270, some of a metal compound having low solubility in water is previously removed from the water to be treated.

The second upstream side precipitating step is performed in the same manner as in the first upstream side precipitating step.

In the case where water to be treated containing Mg ions is treated in the water treatment system 200 of this embodiment, as in the first reference embodiment, the water to be treated is adjusted to a pH at which a magnesium compound is deposited in the second upstream side precipitating section 270, and some of Mg ions in the water to be treated are removed. Subsequently, it is preferable that the water to be treated is adjusted to a pH at which the magnesium compound is soluble on the downstream side of the second upstream side precipitating section 270. Specifically, the pH is adjusted to less than 10. Accordingly, the formation of scales in devices and steps on the downstream side, particularly the second demineralizing section 210 and the second demineralizing step, can be inhibited.

In the case where the second deaerating step is performed after the second upstream side precipitating step, the pH of the water to be treated is adjusted to a pH at which carbonate ions can be present as CO₂, specifically 6.5 or less.

Depending on the properties of the water to be treated, the second deaerating step and the second upstream side precipitating step may be omitted.

In the case where an ion-exchange membrane device is installed, in the water treatment system 200 of the first embodiment, Ca ions and Mg ions in the water to be treated are removed by the ion-exchange membrane device.

In the case where an upstream side crystallizing section is installed, the concentration of gypsum in the water to be treated is reduced in the upstream side crystallizing section through the same steps as in the first reference embodiment.

<Second Scale Inhibitor Supplying Step>

The control section 232 a of the second scale inhibitor supplying section 230 a opens the valve V4 a and supplies a predetermined amount of calcium scale inhibitor to the water to be treated from the tank 231 a. The control section 232 b of the second scale inhibitor supplying section 230 b opens the valve V4 b and supplies a predetermined amount of silica scale inhibitor to the water to be treated from the tank 231 b. The control section 232 a and the control section 232 b adjust the valve opening of the valve V4 a and the valve V4 b, respectively, so that the concentrations of the calcium scale inhibitor and the silica scale inhibitor are predetermined values set according to the properties of the water to be treated.

In the water treatment system 200 of the first embodiment, the pH adjustment of the water to be treated immediately before flowing into the second demineralizing section 210 is optionally performed.

For example, in the configuration of FIG. 10, as a result of the addition of FeCl₃, the water to be treated is adjusted to about pH 5 to 6 and then flows into the second demineralizing section 210 a. As shown in FIG. 3, the solubility of calcium carbonate in water is high when the pH of the water to be treated is 6.5 or less. In addition, as in formula (1), in the above pH range, carbonic acid is present mainly in the form of HCO₃ ⁻ and CO₂ in water. The water to be treated flowing into the second demineralizing section 210 a has a reduced concentration of calcium carbonate. In such a case, it is not necessary to adjust the pH immediately before the second demineralizing section 210 a.

Incidentally, in the case where the pH of the water to be treated, which is to be treated in the second demineralizing step, is adjusted, it is possible that a pH adjusting section having the same configuration as the first pH adjusting section of the first reference embodiment is installed on the upstream of the second demineralizing section 210 a, and the pH-adjusted water to be treated is fed to the second demineralizing section 210 a.

<Second Demineralizing Step>

In the second demineralizing section 210 a, the water to be treated containing the scale inhibitors is treated. In the case where the second demineralizing section 210 a is a reverse osmosis membrane device, the water that has passed through the reverse osmotic membrane is recovered as treated water. The water containing ions and the scale inhibitors is discharged from the non-permeate side of the reverse osmosis membrane as concentrated water (second concentrated water).

As a result of the treatment in the second demineralizing section 210 a, gypsum and silica is concentrated in the second concentrated water. However, the production of scales is suppressed by the calcium scale inhibitor and the silica scale inhibitor.

Also in the case where other demineralizers, such as a capacitive de-ionization device, are used, for example, the water to be treated is separated into treated water and concentrated water having a high concentration of ions (second concentrated water). The second concentrated water is fed toward the second crystallizing section 220 a.

<Second pH Adjusting Step>

In this embodiment, the pH of the water to be treated (second concentrated water) may be adjusted by the second pH adjusting section 240 a between the second demineralizing section 210 a and the second crystallizing section 220 a.

The second pH adjusting section 240 a controls the pH of the second concentrated water to such a value that the function of the calcium scale inhibitor is reduced and gypsum in the second concentrated water can be deposited. The pH meter 243 a measures the pH of the second concentrated water. The control section 242 a adjusts the opening of the valve V6 a so that the value measured by the pH meter 243 a is a predetermined pH control value.

<Second Crystallizing Step>

The second concentrated water is stored in the second crystallizing tank 221 of the second crystallizing section 220 a. The control section 224 a of the second seed crystal supplying section 222 a opens the valve V5 and adds seed crystals of gypsum from the seed crystal tank 223 a to the second concentrated water in the second crystallizing tank 221 a. Although the second concentrated water contains a calcium scale inhibitor, when seed crystals are loaded, gypsum is crystallized, followed by crystal growth.

As shown in FIG. 5, under Condition 1 (pH 6.5), the degree of supersaturation is 460%, and there is no change from the initial degree of supersaturation even after the elapse of 6 hours. Under Condition 1, the scale inhibitor exerts its function to suppress the deposition of gypsum. Meanwhile, under Condition 4 and Condition 2, the degree of supersaturation decreases.

That is, it has been confirmed that even when seed crystals are not loaded, a decrease in pH leads to a decrease in the function of the scale inhibitor, whereby gypsum is crystallized. In addition, according to the results of FIG. 5, the deposition rate increases with a decrease in pH.

In FIG. 6, as a comparison with Condition 5 (pH 4.0), under Condition 7 (pH 4.0), seed crystals pre-immersed in the above calcium scale inhibitor were added to simulated water having added thereto a calcium scale inhibitor, and sulfuric acid was added for pH adjustment. Condition 5 and Condition 7 are otherwise the same, and gypsum deposition experiments were performed under such conditions. Two hours after the pH adjustment, the concentration of Ca in the simulated water was measured by the same technique as in FIG. 3.

As a result, as shown in FIG. 6, the degree of supersaturation was 199% or less both under Condition 5 and Condition 7. From this, it can be said that independent of the immersion time of seed crystals in a calcium scale inhibitor, when the pH is reduced to 4.0, the function of the calcium scale inhibitor is reduced.

In consideration of the effects of the calcium scale inhibitor, the pH of the second concentrated water is adjusted in the second pH adjusting step to 6.0 or less, preferably 5.5 or less, and more preferably 4.0 or less. In particular, when the second concentrated water is adjusted to pH 4.0 or less, the function of the calcium scale inhibitor can be significantly reduced. By adjusting the pH of the second concentrated water to such a value that the scale inhibition function of the calcium scale inhibitor is reduced, crystallization in the second crystallizing section 220 a is promoted. According to the kind of scale inhibitor, the pH range in the second pH adjusting step is suitably determined.

With reference to FIG. 4, in the case where pH is low, silica may exceed the saturation solubility. However, in this embodiment, a silica scale inhibitor is loaded in the second water treatment section. Accordingly, the deposition of silica is suppressed even at low pH. Even when silica is deposited in the second crystallizing tank 221 a, such silica is present as small-diameter particles or floating matters in a colloidal form.

In addition, with reference to FIG. 3, calcium carbonate dissolves in water at pH 6.0 or less.

From the above, high-purity gypsum can be recovered in the second crystallizing tank 221 a of the second water treatment section.

Meanwhile, the second concentrated water in the second crystallizing step may also be adjusted in the second pH adjusting step to a pH at which silica is soluble in the second concentrated water. Accordingly, in the second crystallizing tank 221 a, the deposition of silica from the second concentrated water is suppressed. As a result, in the case where the second concentrated water discharged from the second crystallizing tank 221 a in the second separating section 280 a is classified, the purity of the recovered gypsum can be increased.

<Second Separating Step>

In the case where the second separating section 280 a is installed, the second concentrated water containing solid matters deposited in the second crystallizing tank 221 a is transferred to the second separating section 280 a. In the second concentrated water in the second crystallizing tank 221 a, gypsum deposited by crystallization is present. In addition, the second concentrated water may also contain silica deposited because of an increase in the silica concentration to be equal to or higher than the concentration at which the silica scale inhibitor exerts its function due to the quality change or concentration of the raw water. Silica is present in the second concentrated water as small-diameter particles or floating matters in a colloidal form.

Through the same steps as in the second reference embodiment, the classifier 281 a of the second separating section 280 a performs separation into gypsum having a predetermined size (e.g., having an average particle diameter of 10 μm or more) and a supernatant containing small-diameter precipitates (gypsum, silica). Large-diameter gypsum is further dehydrated by the dehydrator 282 a and recovered. According to this embodiment, high-purity gypsum can be recovered. Some of the recovered gypsum may be circulated through the seed crystal tank 223 a as seed crystals.

In the case where the second separating section 280 a is not installed, gypsum precipitated at the bottom of the second crystallizing tank 221 a of the second crystallizing section 220 a is discharged from the second crystallizing tank 221 a. The supernatant in the second crystallizing tank 221 a is fed to the second precipitating section 250 a.

<Second Precipitating Step>

The supernatant (second concentrated water) in the second crystallizing section 220 a or the supernatant (second concentrated water) discharged from the second separating section 280 a is fed to the second precipitating section 250 a.

In the second precipitating step, in the same manner as in the first precipitating step described in the first reference embodiment, calcium carbonate and metal compounds in the second concentrated water are removed in the second precipitating tank 251 a and the second filtration device 252 a.

In the second precipitating step, it is also possible that in the same manner as in the first precipitating step, at least one of seed crystals of silica and a precipitant for silica is added to the second precipitating tank 251 a to remove silica from the second concentrated water.

In the case where the treatment is performed in several stages as shown in FIG. 10, the second concentrated water that has passed through the second filtration device 252 a of the second water treatment section of the previous stage flows into the water treatment section of the subsequent stage as water to be treated. In the water treatment section of the subsequent stage, the steps from the second scale inhibitor supplying step to the second precipitating step mentioned above are performed.

<Downstream Side Demineralizing Step>

The second concentrated water that has passed through the second precipitating section 250 b located on the most downstream of the water to be treated is treated in the downstream side demineralizing section 60. The water that has passed through the downstream side demineralizing section 60 is recovered as water to be treated. The concentrated water in the downstream side demineralizing section 60 is discharged out of the system.

Also in this embodiment, an evaporator (not shown) may be installed on the downstream on the concentrated-water side of the downstream side demineralizing section 60.

In the first embodiment, in the case where the second concentrated water is adjusted in the second pH adjusting step to a pH at which the function of the calcium scale inhibitor is reduced, as a third pH adjusting step (a pH adjusting step for achieving the calcium scale inhibition function), the pH of the second concentrated water may be adjusted after the second crystallizing step in order for the calcium scale inhibitor to exert its function. Specifically, the pH is preferably adjusted to 4.0 or more, preferably 5.5 or more, and more preferably 6.0 or more. The third pH adjusting step is performed after the second crystallizing step and before the second demineralizing step, or after the second crystallizing step and before the downstream side demineralizing step.

In the water treatment system 200 of this embodiment, in order to perform the third pH adjusting step, a third pH adjusting section (not shown in FIG. 10) having the same configuration as the second pH adjusting section is installed between the second crystallizing section and the second demineralizing section immediately thereafter (in FIG. 10, between the second crystallizing section 220 a and the second demineralizing sections 210 b, particularly the second precipitating section 250 a and the second demineralizing section 210 b). In addition, a third pH adjusting section (not shown in FIG. 10) having the same configuration as the second pH adjusting section is installed between the second precipitating section 250 b and the downstream side demineralizing section 60 on the most downstream. Accordingly, even in the case where the second concentrated water is treated in the downstream side demineralizing step, and the concentration of Ca ions is high on the concentrated-water side, the formation of scales can be suppressed by the function of the calcium scale inhibitor.

In the water treatment system 200 of this embodiment, silica is concentrated by the treatment in the second water treatment section. When the concentration of silica in the second concentrated water is equal to or higher than the concentration at which the silica scale inhibitor effectively works, scales of silica may be formed from the second concentrated water. For example, in the case where FLOCON260 is used as a silica scale inhibitor, the scale-production-inhibiting effect can be obtained at a silica concentration up to about 200 mg/L. Therefore, the number of stages of the second water treatment sections is determined so that silica is concentrated to the concentration at which the silica scale inhibitor can exert its effect.

By using the water treatment system 200 of the first embodiment, water to be treated containing ions can be treated with high water recovery.

In particular, in the first embodiment, gypsum is mainly deposited in the second crystallizing section 220. Accordingly, the gypsum recovery in the second crystallizing section 220 is high, and the number of moles of ions fed to the downstream side is further reduced. In addition, the purity of the gypsum recovered in the second crystallizing section 220 can be increased.

Third Reference Embodiment

FIG. 11 is a schematic diagram of a water treatment system of the third reference embodiment of the present invention. In FIG. 11, the same configurations as in the first reference embodiment, the second reference embodiment, and first embodiment are indicated with the same reference numerals.

In the water treatment system 300 of the third reference embodiment, the water treatment section described in the first reference embodiment is installed. On the downstream side of the water to be treated of this water treatment section, the water treatment section described in the first embodiment is installed.

In the water treatment system 300 of FIG. 11, a first separating section 180 is installed on the downstream side of the first crystallizing section 20. In addition, a second separating section 280, which is the same as the first separating section 180, is installed on the downstream side of the second crystallizing section 220.

A downstream side demineralizing section 60 is installed on the downstream side of the water to be treated of the second crystallizing section 220 located on the most downstream.

The water treatment system 300 of the third reference embodiment includes the first upstream side precipitating section 70 described in the first reference embodiment on the upstream side of the first scale inhibitor supplying section 30 and the first pH adjusting section 40 which are located on the most upstream of the water to be treated.

Further, the water treatment system 300 of the third reference embodiment includes a first deaerating section 73, which is the same as in the first reference embodiment, on the upstream side of the first upstream side precipitating section 70 as shown in FIG. 11. The first deaerating section 73 may also be installed on the downstream side of the water to be treated of the first upstream side precipitating section 70 and on the upstream side of the first scale inhibitor supplying section 30 and the first pH adjusting section 40.

Incidentally, a deaerating section having the same configuration as the first deaerating section 73 may be installed in the flow path between the first demineralizing section 10 and the first crystallizing section 20, in the flow path between the first crystallizing section 20 and the first precipitating section 50, in the flow path between the second crystallizing section 220 and the second precipitating section 250, and in the flow path between the first precipitating section 50 and the second demineralizing section 210.

Also in the water treatment system 300 of this reference embodiment, an ion-exchange equipment (not shown) and an upstream side crystallizing section (not shown) may be provided on the upstream of the first scale inhibitor supplying section 30 and the first pH adjusting section 40.

In FIG. 11, the water treatment sections from the first scale inhibitor supplying section 30 to the first precipitating section 50 and from the second scale inhibitor supplying section 230 to the second precipitating section 250 are each shown as one stage. However, it is also possible that for each section, two or more stage of water treatment sections are connected.

In the water treatment system 300 of the third reference embodiment, first, water to be treated is treated by the water treatment process described in the first reference embodiment and the second reference embodiment. First concentrated water after being treated by the process of the first reference embodiment and the second reference embodiment is treated as water to be treated through the steps form the second scale inhibitor supplying step to the second precipitating step described in the first embodiment.

Second concentrated water that has passed through the second precipitating section 250 on the most downstream is treated in the downstream side demineralizing section 60. The water that has passed through the downstream side demineralizing section 60 is recovered as treated water. The concentrated water in the downstream side demineralizing section 60 is discharged out of the system.

Also in this reference embodiment, an evaporator (not shown) may be installed on the downstream on the concentrated-water side of the downstream side demineralizing section 60.

In the third reference embodiment, in the case where the second concentrated water is adjusted to a pH at which the function of the calcium scale inhibitor is reduced in the second pH adjusting step, the third pH adjusting step described in the first embodiment may be performed.

Fourth Reference Embodiment

FIG. 12 is a schematic diagram of a water treatment system of the fourth reference embodiment of the present invention. In FIG. 12, the same configurations as in the first reference embodiment, the second reference embodiment, and first embodiment are indicated with the same reference numerals.

In the water treatment system 400 of the fourth reference embodiment, the water treatment section described in the first embodiment is installed. On the downstream side of the water to be treated of this water treatment section, the water treatment section described in the first reference embodiment is installed.

In the water treatment system 400 of FIG. 12, a first separating section 180 and a second separating section 280 are installed.

A downstream side demineralizing section 60 is installed on the downstream side of the water to be treated of the first crystallizing section 20 located on the most downstream.

The water treatment system 400 of the fourth reference embodiment includes the second upstream side precipitating section 270 described in the first embodiment on the upstream side of the second scale inhibitor supplying section 230 located on the most upstream of the water to be treated.

Further, the water treatment system 400 of the fourth reference embodiment has a second deaerating section 273, which is the same as in the first embodiment, on the upstream side of the second upstream side precipitating section 270 as shown in FIG. 12. The second deaerating section 273 may be installed on the downstream side of the water to be treated of the second upstream side precipitating section 270 and on the upstream side of the second scale inhibitor supplying section 230.

Incidentally, a deaerating section having the same configuration as the second deaerating section 273 may be installed in the flow path between the second demineralizing section 210 and the second crystallizing section 220, in the flow path between the first crystallizing section 20 and the first precipitating section 50, in the flow path between the second crystallizing section 220 and the second precipitating section 250, and in the flow path between the second precipitating section 250 and the first demineralizing section 10.

Also in the water treatment system 400 of this reference embodiment, an ion-exchange equipment (not shown) and an upstream side crystallizing section (not shown) may be provided on the upstream of the second scale inhibitor supplying section 230.

In the water treatment system 400 of this reference embodiment shown in FIG. 12, a first separating section 180 and a second separating section 280 may be installed on the downstream side of the first crystallizing tank 21 and the second crystallizing tank 221, respectively.

In FIG. 12, the water treatment sections from the second scale inhibitor supplying section 230 to the second precipitating section 250 and from the first scale inhibitor supplying section 30 to the first precipitating section 50 are each shown as one stage. However, it is also possible that for each section, two or more stage of water treatment sections are connected.

In the water treatment system 400 of the fourth reference embodiment, first, water to be treated is treated by the water treatment process described in the first embodiment. Second concentrated water after being treated by the process of the first embodiment is treated as water to be treated through the steps form the first scale inhibitor supplying step to the first precipitating step described in the first reference embodiment and the second reference embodiment.

First concentrated water that has passed through the first precipitating section 50 on the most downstream is treated in the downstream side demineralizing section 60. The water that has passed through the downstream side demineralizing section 60 is recovered as treated water. The concentrated water in the downstream side demineralizing section 60 is discharged out of the system.

Also in this reference embodiment, an evaporator (not shown) may be installed on the downstream on the concentrated-water side of the downstream side demineralizing section 60.

In the fourth reference embodiment, in the case where the second concentrated water is adjusted to a pH at which the function of the calcium scale inhibitor is reduced in the second pH adjusting step, the third pH adjusting step described in the first embodiment may be performed.

Also by the water treatment system 300 of the third reference embodiment and the water treatment system 400 of the fourth reference embodiment, water to be treated containing ions can be treated with high water recovery.

In particular, the fourth reference embodiment is configured such that gypsum is mainly deposited in the second crystallizing section 220 on the upstream side of the water to be treated. Accordingly, the gypsum recovery in the second crystallizing section 220 is high, and the number of moles of ions fed to the downstream side is further reduced. Further, the purity of the gypsum recovered in the second crystallizing section 220 can be increased.

Fifth Reference Embodiment

According to the fifth reference embodiment of the present invention, the amount of seed crystals of gypsum to be supplied to the first crystallizing tank 21 and the second crystallizing tank 221 in the first to fourth reference embodiments is controlled. The configuration that controls the amount of seed crystals to be supplied to the first crystallizing tank 21 will be described with reference to FIG. 13. The same configuration is also applied to the second crystallizing tank 221.

In the fifth reference embodiment, a first pH measuring section 543 that measures the pH of the first concentrated water in the first crystallizing tank 21 is installed. The first pH measuring section 543 may be installed in the flow path that connects the first demineralizing section 10 and the first crystallizing tank 21, or may also be directly installed in the first crystallizing tank 21. The first pH measuring section 543 is connected to the control section 24 of the first seed crystal supplying section 22.

In the fifth reference embodiment, as shown in FIG. 13, a section 540 for adjusting pH used for controlling the seed crystal supply is installed. The section 540 for adjusting pH used for controlling the seed crystal supply includes a tank 541, a control section 542, and a valve V7. The first pH measuring section 543 is connected to the control section 542 of the section 540 for adjusting pH used for controlling the seed crystal supply. The section 540 for adjusting pH used for controlling the seed crystal supply controls the pH of the first concentrated water in the first crystallizing tank 21 to a predetermined value based on the value measured by the first pH measuring section 543.

Incidentally, in the case where the amount of seed crystals of gypsum to be supplied to the second crystallizing tank 221 is controlled, the pH meter 243 a described in the first embodiment corresponds to the second pH measuring section, and the control section 242 of the second pH adjusting section corresponds to the control section 542.

Seed crystals stored in the seed crystal tank 23 of the first seed crystal supplying section 22 may be new chemicals. However, in the case where a first separating section 180 is installed, the seed crystal tank 23 may also store gypsum separated by the classifier 181, whose particle diameter is equal to or greater than a predetermined particle diameter, or gypsum after being dehydrated by the dehydrator 182.

The control of the amount of seed crystals to be supplied in the fifth reference embodiment is performed through the following steps. Hereinafter, the case where the amount of seed crystals to be supplied is constantly controlled during continuous operation will be explained as an example.

The first pH measuring section 543 measures the pH of the first concentrated water in the first crystallizing tank 21. The measured pH value is sent to the control section 24 of the seed crystal supplying section 22.

The control section 24 stores the pH range where the scale inhibition function of a calcium scale inhibitor is reduced. Specifically, as described for the second crystallizing step, the pH range where the scale inhibition function of a calcium scale inhibitor is reduced is 6.0 or less, preferably 5.5 or less, and more preferably 4.0 or less.

The control section 24 compares the value measured by the first pH measuring section 543 with the above pH range. In the case where the measured value is within the above pH range, the control section 24 reduces the opening of the valve V3 to reduce the amount of seed crystals of gypsum to be supplied. In the case where the measured value is greater than the above pH range, the control section 24 increases the opening of the valve V3 to increase the amount of seed crystals of gypsum to be supplied.

Gypsum is deposited when seed crystals are present. However, in the case where the calcium scale inhibitor exerts its function, the crystallization rate is low. Accordingly, the amount of seed crystals is increased to promote crystallization. Meanwhile, in the case where the function of the calcium scale inhibitor is reduced, a sufficient crystallization rate can be obtained even when the amount of seed crystals is small.

In this way, by adjusting the amount of seed crystals to be supplied according to the pH, the amount of seed crystals used can be reduced.

In this reference embodiment, it is also possible that the pH is regularly measured during continuous operation, and seed crystals are supplied intermittently. Alternatively, it is also possible that the time-dependent variation of pH is obtained at the time of the test run of the system, for example, and the amount of seed crystals to be supplied is increased or decreased based on the obtained time-dependent variation.

Sixth Reference Embodiment

The sixth reference embodiment of the present invention is a water treatment system 600 provided with at least either a first separating section 180 or a second separating section 280. The water treatment system 600 differs from the fifth reference embodiment in that gypsum separated in the separating section is directly supplied to a first crystallizing tank or a second crystallizing tank as seed crystals.

The configuration that controls the amount of seed crystals to be supplied to the first crystallizing tank 21 in this reference embodiment will be described with reference to FIG. 14. The same configuration is also applied to the second crystallizing tank 221.

In FIG. 14, a first circulation line 601, which performs transfer so that some of the gypsum sedimented at the bottom of the classifier 181 of the first separating section 180 is supplied directly to the first crystallizing tank 21, is installed. In addition, a second circulation line 602, which performs transfer so that some of the gypsum after being dehydrated by the dehydrator 182 is supplied directly to the first crystallizing tank 21, is installed. A valve V8 is installed in the first circulation line 601, and a valve V9 is installed in the second circulation line 602. Incidentally, this reference embodiment may also be configured such that either the first circulation line 601 or the second circulation line 602 is installed.

The control section 610 is connected to a first pH measuring section 543, which is the same as in the fifth reference embodiment, the valve V8, and the valve V9.

The control of the amount of seed crystals to be supplied in the sixth reference embodiment is performed through the following steps. Hereinafter, the case where the amount of seed crystals to be supplied is constantly controlled during continuous operation will be explained as an example.

The first pH measuring section 543 measures the pH of the first concentrated water in the first crystallizing tank 21. The measured pH value is sent to the control section 610.

The control section 610 stores the pH range where the scale inhibition function of a calcium scale inhibitor is reduced. Through the same steps as in the fifth reference embodiment, the control section 610 compares the value measured by the first pH measuring section 543 with the above pH range to adjust the opening of the valve V8 and the valve V9.

In the fifth reference embodiment and the sixth reference embodiment, a seed crystal concentration measuring section (not shown) that measures the concentration of gypsum seed crystals in the first concentrated water in the first crystallizing tank 21 may be installed in the first crystallizing tank 21. The seed crystal concentration measuring section measures the concentration of seed crystals in the first crystallizing tank 21. The measured concentration value is sent to the control section 24 or the control section 610. The control section 24 or the control section 610 stores the threshold for the concentration of seed crystals, and increases the amount of seed crystals to be supplied in the case where the concentration of seed crystals is equal to or less than the threshold.

As a modification of the fifth reference embodiment and the sixth reference embodiment, a first concentration measuring section (not shown) is installed on the downstream side of the first crystallizing tank 21 and on the upstream side of the first precipitating section 50. In the case where the first separating section 180 is provided, the first concentration measuring section is preferably installed on the downstream side of the first separating section 180, but may also be installed on the upstream side of the first separating section 180. The first concentration measuring section is connected to the control section 24 or the control section 610.

In the case of the second crystallizing tank 221, a second concentration measuring section is installed in place of the first concentration measuring section.

The first concentration measuring section measures at least one of the concentration of Ca ions and the concentration of sulfate ions in the first concentrated water discharged from the first crystallizing tank 21. The measured concentration is sent to the control section 24 or the control section 610.

The concentration of Ca ions and the concentration of sulfate ions measured by the first concentration measuring section depend on the crystallization rate in the first crystallizing tank 21. In the case where the residence time is the same, lower concentrations of Ca ions and sulfate ions lead to a higher crystallization rate.

The control section 24 and the control section 610 store the threshold for at least one of the concentration of Ca ions and the concentration of sulfate ions.

In the case where at least one of the concentration of Ca ions and the concentration of sulfate ions measured by the first concentration measuring section is equal to or higher than the threshold, the control section 24 increases the opening of the valve V3 to increase the amount of seed crystals to be supplied. In the case where at least one of the concentration of Ca ions and the concentration of sulfate ions measured by the first concentration measuring section is lower than the threshold, the control section 24 reduces the opening of the valve V3 to reduce the amount of seed crystals to be supplied.

In the case where at least one of the concentration of Ca ions and the concentration of sulfate ions measured by the first concentration measuring section is equal to or higher than the threshold, the control section 610 increases the opening of the valve V8 and the valve V9 to increase the amount of seed crystals to be supplied. In the case where at least one of the concentration of Ca ions and the concentration of sulfate ions measured by the first concentration measuring section is lower than the threshold, the control section 610 reduces the opening of the valve V8 and the valve V9 to reduce the amount of seed crystals to be supplied.

Also in the case of the second crystallizing tank 221, the amount of seed crystals to be supplied is controlled through the same steps as above.

In this way, by controlling the amount of seed crystals to be supplied depending on at least one of the concentration of Ca ions and the concentration of sulfate ions after the crystallizing step, the amount of seed crystals used can be reduced.

Seventh Reference Embodiment

FIG. 15 is a partial schematic diagram of a water treatment system of the seventh reference embodiment of the present invention. In FIG. 15, the same configurations as in the first reference embodiment, the second reference embodiment, and first embodiment are indicated with the same reference numerals.

The water treatment system 700 of FIG. 15 is configured such that the gypsum separated from the first concentrated water in the first crystallizing section 20 in the water treatment system of the fourth reference embodiment is recovered and supplied to the second crystallizing tank 221 of the second crystallizing section 220. Also in the water treatment system of the third reference embodiment, the same configuration can be employed.

As described in the first reference embodiment, the pH of concentrated water (first concentrated water) in the first crystallizing tank 21 of the first crystallizing section 20 is not particularly limited. However, in terms of operation cost, it is more advantageous to perform the first crystallizing step without changing the pH from the first demineralizing step. In this case, the first crystallizing step is performed at a pH at which silica is soluble (10 or more), but the solubility of calcium carbonate is low in this pH range.

Meanwhile, as described in the first embodiment, in the second crystallizing section 220 (second crystallizing step), gypsum is crystallized in a still lower pH range. At the pH range in the second crystallizing step (6.0 or less, more preferably 4.0 or less), calcium carbonate is soluble in water. When gypsum containing calcium carbonate recovered in the first crystallizing section 20 is supplied to the second crystallizing tank 221 of the second crystallizing section 220, calcium carbonate, which is an impurity, dissolves in the second concentrated water, and gypsum is present as a solid in the second concentrated water. By using the water treatment system 700 of the seventh reference embodiment, water to be treated can be treated with high water recovery, and also high-purity gypsum can be recovered.

Eighth Reference Embodiment

FIG. 16 is a partial schematic diagram of a water treatment system of the eighth reference embodiment of the present invention. In FIG. 16, the same configurations as in the second reference embodiment are indicated with the same reference numerals.

Incidentally, the eighth reference embodiment will be described hereinafter using a water treatment process including a first separating step and a water treatment system including a first separating section. However, the same configuration is also applicable to the case of a second separating step and a second separating section.

In FIG. 16, the water treatment system 800 includes, for one first crystallizing section 20, two or more classifiers (first classifiers) 181 in the flow direction of the water to be treated. In FIG. 16, two first classifiers 181 a and 181 b are installed. The size of gypsum to be separated is different between the first classifier 181 a located on the most upstream and the first classifier 181 b located on the downstream side. In this reference embodiment, the size of gypsum to be separated by the first classifier 181 b is smaller than that of gypsum to be separated by the first classifier 181 a. For example, the first classifier 181 a is a classifier that separates particles having an average particle diameter of 10 μm or more, and the first classifier 181 b is a classifier that separates particles having an average particle diameter of 5 μm or more.

In the case where three or more first classifiers 181 are installed, they are designed such that the size of gypsum to be separated by each first classifier 181 decreases in the direction from the upstream side toward the downstream side. The number of first classifiers installed in the flow direction of the water to be treated and the particle diameter of solid matters that can be separated by each classifier are suitably determined in consideration of the water recovery, gypsum recovery, treatment cost, etc.

In the water treatment system 800 of the eighth reference embodiment, the following treatment is performed in the first separating step.

In the first classifier 181 a located on the most upstream, gypsum having an average particle diameter of 10 μm or more is classified and sedimented at the bottom of the first classifier 181 a. The sedimented gypsum is discharged from the first classifier 181 a and fed to the dehydrator 182. The supernatant in the first classifier 181 a is fed to the first classifier 181 b on the downstream side. This supernatant mainly contains particles having a particle diameter of less than 10 μm (gypsum, calcium carbonate, silica, etc.).

In the first classifier 181 b located on the downstream side, gypsum having an average particle diameter of 5 μm or more is classified and sedimented at the bottom of the first classifier 181 b. The supernatant in the first classifier 181 b is fed to the first precipitating section 50.

The sedimented gypsum is discharged from the first classifier 181 b. The discharged gypsum is fed to the first crystallizing tank 21 through a solid matter circulation line 801 and supplied into the first concentrated water in the first crystallizing tank 21.

The circulated gypsum functions as seed crystals in the first crystallizing tank 21, and the circulated gypsum is crystallized, followed by crystal growth. The crystal-grown circulated gypsum having an average particle diameter of 10 μm or more is fed from the first crystallizing tank 21 to the first classifier 181 a together with the first concentrated water, then separated from the first concentrated water by the first classifier 181 a, and transferred to the dehydrator 182.

The supernatant in the first classifier 181 b contains particles having a relatively small diameter of less than 5 μm, such as those having a particle diameter of about 2 to 3 μm. In particular, in the early stage of the operation of the water treatment system (immediately after start-up, etc.), gypsum is discharged from the first crystallizing tank 21 before it grows to a sufficient size in the first crystallizing tank 21, and an increased amount of gypsum flows into the first precipitating tank 51. In such a case, a large amount of gypsum is contained in the precipitate in the first precipitating tank 51. Thus, in this reference embodiment, it is also possible that a circulation line 802 that connects the bottom of the first precipitating tank 51 to the first crystallizing tank 21 is provided, and solid matters containing gypsum precipitated at the bottom of the first precipitating tank 51 are circulated through the first crystallizing tank 21.

According to this reference embodiment, the amount of gypsum recovered in the first separating section is increased, and also the water content of the recovered gypsum can be reduced. The use of the water treatment processes and the water treatment system of this reference embodiment leads to the reduction of the amount of gypsum particles having a relatively small diameter flowing out to the downstream side. Accordingly, the water recovery can be improved, and also the amount of waste resulting from the water treatment can be reduced.

REFERENCE SIGNS LIST

-   1, 100, 200, 300, 400, 500, 600, 700, 800: Water treatment system -   10: First demineralizing section -   20: First crystallizing section -   21: First crystallizing tank -   22: First seed crystal supplying section -   23, 223: Seed crystal tank -   24, 32, 42, 224, 232, 242, 542, 610: Control section -   30: First scale inhibitor supplying section -   31, 41, 231, 241, 541: Tank -   40: First pH adjusting section -   43, 243: pH meter -   50: First precipitating section -   51: First precipitating tank -   52: First filtration device -   60: Downstream side demineralizing section -   70: First upstream side precipitating section -   71: Precipitating tank -   72: Filtration device -   73: First deaerating section -   180: First separating section -   181, 181 a, 181 b, 281: Classifier -   182, 282: Dehydrator -   210: Second demineralizing section (demineralizing section) -   220: Second crystallizing section (crystallizing section) -   221: Second crystallizing tank (crystallizing tank) -   222: Second seed crystal supplying section (seed crystal supplying     section) -   230: Second scale inhibitor supplying section (scale inhibitor     supplying section) -   240: Second pH adjusting section (pH adjusting section) -   250: Second precipitating section (precipitating section) -   251: Second precipitating tank (precipitating tank) -   252: Second filtration device (filtration device) -   280: Second separating section (separating section) -   540: section for adjusting pH used for controlling the seed crystal     supply -   543: First pH measuring section -   601: First circulation line -   602: Second circulation line -   801, 802: Solid matter circulation line 

The invention claimed is:
 1. A water treatment process, comprising: a scale inhibitor supplying step of supplying a calcium scale inhibitor which is a scale inhibitor for inhibiting the deposition of a scale containing calcium and a silica scale inhibitor which is a scale inhibitor for inhibiting the deposition of silica to water containing Ca²⁺ ions, SO₄ ²⁻ ions, carbonate ions and silica; a demineralizing step of separating the water into concentrated water in which the Ca²⁺ ions, the SO₄ ²⁻ ions, the carbonate ions and the silica are concentrated and treated water after the scale inhibitor supplying step; a crystallizing step of supplying seed crystals of gypsum to the concentrated water so that gypsum is crystallized from the concentrated water; and a pH adjusting step of adjusting the concentrated water to a pH of 6.0 or less, at which a scale inhibition function of the calcium scale inhibitor is reduced, thereby promoting the deposition of the gypsum in the crystallizing step.
 2. The water treatment process according to claim 1, comprising a downstream side demineralizing step of separating concentrated water after the crystallizing step into concentrated water and treated water, and recovering the separated treated water.
 3. The water treatment process according to claim 2, wherein moisture is evaporated from the concentrated water in the downstream side demineralizing step, so that a solid in the concentrated water is recovered.
 4. The water treatment process according to claim 1, wherein, after the crystallizing step, the concentrated water after the adjustment of the pH in the pH adjusting step is adjusted to a pH of more than 6.0 at which the calcium scale inhibitor exhibits its function.
 5. The water treatment process according to claim 1, comprising a pH adjusting step of adjusting the concentrated water to a pH of 10 or more at which the silica is soluble in the crystallizing step.
 6. The water treatment process according to claim 1, comprising a upstream side precipitating step of precipitating at least calcium carbonate from the water so that the concentration of the calcium carbonate in the water is reduced, before the scale inhibitor supplying step.
 7. The water treatment process according to claim 6, comprising a deaerating step of removing CO₂ from the water before the upstream side precipitating step or after the upstream side precipitating step and before the scale inhibitor supplying step.
 8. The water treatment process according to claim 6, wherein, when the water contains Mg²⁺ ions, the water in the upstream side precipitating step is adjusted to a pH of 10 or more at which a magnesium compound is deposited so that the concentration of the Mg²⁺ ions is reduced, and wherein, after the upstream side precipitating step, the water is adjusted to a pH of less than 10 at which the magnesium compound is soluble.
 9. The water treatment process according to claim 1, wherein the water contains metal ions; and wherein the process comprises a precipitating step of precipitating at least one of calcium carbonate and a metal compound so that the concentration of at least one of the calcium carbonate and the metal ions is reduced from the concentrated water, after the crystallizing step.
 10. The water treatment process according to claim 9, wherein at least one of seed crystals of silica and a precipitant for the silica is supplied to the concentrated water in the precipitating step.
 11. The water treatment process according to claim 10, wherein, when the water contains Mg²⁺ ions, the amount of the precipitant for the silica to be supplied is adjusted according to the concentration of the Mg²⁺ ions.
 12. The water treatment process according to claim 9, wherein, when the water contains Mg²⁺ ions, the concentrated water in the precipitating step is adjusted to a pH of 10 or more at which a magnesium compound is deposited so that the concentration of the Mg²⁺ ions is reduced, and wherein, after the precipitating step, the concentrated water is adjusted to a pH of less than 10 at which the magnesium compound is soluble.
 13. A water treatment system, comprising: a scale inhibitor supplying section that supplies a calcium scale inhibitor which is a scale inhibitor for inhibiting the deposition of a scale containing calcium and a silica scale inhibitor which is a scale inhibitor for inhibiting the deposition of silica to water containing Ca²⁺ ions, SO₄ ²⁻ ions, carbonate ions and silica; a demineralizing section that is installed on a downstream side of the scale inhibitor supplying section and separates the water into concentrated water in which the Ca²⁺ ions, the SO₄ ²⁻ ions, the carbonate ions and the silica are concentrated and treated water; a crystallizing section including a crystallizing tank that is installed on a downstream side of the demineralizing section and crystallizes gypsum from the concentrated water and a seed crystal supplying section that supplies seed crystals of gypsum to the crystallizing tank; and a pH adjusting section that is installed on a downstream side of the demineralizing section and supplies a pH adjuster to the concentrated water to adjust the pH of the concentrated water to 6.0 or less such that a scale inhibition function of the calcium scale inhibitor is reduced, and the precipitation of the gypsum is promoted.
 14. The water treatment system according to claim 13, comprising, on a downstream side of the crystallizing section, a downstream side demineralizing section that separates the concentrated water discharged from the crystallizing section into concentrated water and treated water.
 15. The water treatment system according to claim 14, comprising, on a downstream side of the concentrated water in the downstream side demineralizing section, an evaporator that evaporates moisture from the concentrated water to recover the solids in the concentrated water.
 16. The water treatment system according to claim 13, comprising, on a downstream side of the crystallizing section, a pH adjusting section for achieving scale inhibition function that supplies a pH adjuster to the concentrated water after the adjustment of the pH in the pH adjusting section to adjust the pH of the concentrated water to more than 6.0 such that the calcium scale inhibitor achieves a function.
 17. The water treatment system according to claim 13, comprising a pH adjusting section that is installed on a downstream side of the demineralizing section and supplies a pH adjuster to the concentrated water to adjust the pH of the concentrated water to 10 or more such that the silica is soluble in the concentrated water in the crystallizing section.
 18. The water treatment system according to claim 13, comprising, on an upstream side of the scale inhibitor supplying section, a upstream side precipitating section that precipitates at least calcium carbonate from the water so that the concentration of the calcium carbonate in the water is reduced.
 19. The water treatment system according to claim 18, comprising a deaerating section that removes CO₂ from the water on an upstream side of the upstream side precipitating section or on a downstream side of the upstream side precipitating section and on an upstream side of the scale inhibitor supplying section.
 20. The water treatment system according to claim 18, wherein, when the water contains Mg²⁺ ions, the water in the upstream side precipitating section is adjusted to a pH of 10 or more at which a magnesium compound is deposited so that the concentration of the Mg²⁺ ions is reduced, and wherein, on a downstream side of the upstream side precipitating section, the water is adjusted to a pH of less than 10 at which the magnesium compound is soluble.
 21. The water treatment system according to any claim 13, wherein the water contains metal ions; and wherein the system comprises, on a downstream side of the crystallizing section, a precipitating section that precipitates at least one of calcium carbonate and a metal compound.
 22. The water treatment system according to claim 21, wherein at least one of seed crystals of silica and a precipitant for the silica is supplied to the precipitating section.
 23. The water treatment system according to claim 21, wherein, when the water contains Mg²⁺ ions, the amount of the precipitant for the silica to be supplied is adjusted according to the concentration of the Mg²⁺ ions in the precipitating section.
 24. The water treatment system according to claim 21, wherein, when the water contains Mg²⁺ ions, the concentrated water in the precipitating section is adjusted to a pH of 10 or more at which a magnesium compound is deposited so that the concentration of the Mg²⁺ ions is reduced, and wherein, on a downstream side of the precipitating section, the concentrated water is adjusted to a pH of less than 10 at which the magnesium compound is soluble. 